Chelating/complexing dip in bright plating of brass

ABSTRACT

PROCESS OF NICKEL BRASS-TYPE COPPER ALLOYS. THE CONVENTIONAL PLATING CYCLE STEPS OF (1) SOAK CLEAN, (2) ELCTROCLEAN (AS SINGLE OR TWO SEPARATE STEP OPERATION), (3) ACID DIP AND (4) THEN ELECTROPLATE, ARE MODIFIED BY INCORPORATING AN ADDITIONAL STEP CONSISTING IN IMMERSING THE COPPER ALLOY SUBSTRATE IN AN AQUEOUS CHELATING/COMPLEXING SOLUTION AFTER THE SOAK CLEAN BUT BFORE THE ACID DIP STEPS. THE PURPOSE IS TO ELIMINATE VISIBLE IRREGULARITIES IN THE NICKEL DEPOSIT, AS WELL AS TO IMPROVE ADHESION OF THE PLATE TO THE SUBSTRATE. THE PREFERRED CHELATING/COMPLEXING MATERIALS IN AQUEOUS SOLUTION INCLUDE THE ALKALI METALS SALTS OF POLYCARBOXYLIC ACIDS, SUCH AS GLUCONIC, TARTARIC, HEPTONIC, OXALIC AND CITRIC; THE ALKALI METALS SALTS OF POLYPHOSPHORIC ACIDS; THE ALKALI METAL SALTS OF LOWER ALKYL-SUBSTITUTED AMINO ACETIC ACIDS AND HYDROXY AMINO ACIDS; AND COMPATIBLE COMBINATIONS OF SUCH AGENTS.

United States Patent O US. Cl. 204-32 R 8 Claims ABSTRACT OF THEDISCLOSURE Process of nickel plating brass-type copper alloys. Theconventional plating cycle steps of (1) soak clean, (2) electroclean (asa single or two separate step operation),

(3) acid dip and (4) then electroplate, are modified by incorporating anadditional step consisting in immersing the copper alloy substrate in anaqueous chelating/complexing solution after the soak clean but beforethe acid dip steps. The purpose is to eliminate visible irregularitiesin the nickel deposit, as well as to improve adhesion of the plate tothe substrate. The preferred chelating/complexing materials in aqueoussolution include the alkali metal salts of polycarboxylic acids, such asgluconic, tartaric, heptonic, oxalic and citric; the alkali metal saltsof polyphosphoric acids; the alkali metal salts of loweralkyl-substituted amino acetic acids and hydroxy amino acids; andcompatible combinations of such agents.

SUMMARY OF THE INVENTION Field of Invention The invention is directed toimproving the surface uniformity and specular appearance as well as theadhesion of nickel electroplates, more particularly bright nickelelectroplates, on brass-type copper alloy substrates.

PRIOR ART Brass and related copper-tin-zinc-lead alloys are by far themost difficult metal substrates to electroplate properly in order toproduce a finish which is free of visual or optical irregularities suchas clouding, haze or striation, as well as being free of mechanicalirregularities such as poor adhesion and blistering. Nickel electroplateand especially decorative bright nickel deposits are most troublesome inrespect to clouding, etc. and poor adhesion. It is well known thatcopper alloys containing zinc, lead and tin are susbstantially moretroublesome in this respect than is copper itself when used as a base orsubstrate metal; yet for better physical properties, such as castabilityor free machining, the alloying components are a practical necessity inthe substrate.

Various attempts have been made heretofore to over come clouding, haze,striation, as well as poor adhesion, in nickel plated brass substrates.The problem of objectionable patterns in the finished surface is oftenemphasized or exaggerated by final chromium plating of nickel platedbuffed brass parts, and frequently these do not appear until the laststage of manufacture has been completed. Such defects in surfacecharacteristics naturally cause the parts to lose esthetic appeal andrequire their rejection even if mechanically the finish is satisfactory.

Attempts to improve the results have been directed toward varyingconcentrations, temperatures, current densities, immersion times, aswell as compositions, in the various treatment solutions generallyemployed in the normal pretreatment and electroplating operations. Sincethe number of operations involved in the normal cycle is substantial,the permutations and combinations possible in the way of changes thatcan be made become quite complex, and their coordination is largely amatter of empirical determination. This will be more evident from thefollowing outline of steps usually involved in the typical preparationand plating cycle for a brass or similar brasstype copper alloysubstrate:

Conventional preparation and plating cycle steps (a) Soak clean, 1 to 5minutes at 160-200 F. in a proprietary alkaline cleaner, such as amoderately alkaline solution of borax, pyrophosphate, soap, ethoxylatednonionic surfactant, a small amount of organic chelating agent,dispersing agent and suitable water soluble solvent, such as ethyleneglycol monobutyl ether. Such a cleaner is conventionally used at aconcentration, for example, of about 8 ozs./gal.

(b) Following a thorough water rinse, electroclean the substrate in anysuitable proprietary composition, such as a solution of soda ash, sodiummetasilicate glassy phosphate, sodium hydroxide and a minor amount ofanionic' surfactant at a concentration of about 8 ozs./gal. for 10 to 30seconds at -160 F. wherein the part is made anodic or cathodic or acombination of cathodic and anodic, using potentials to give a currentdensity of 25 ASP.

(0) Again following a thorough water rinse, the electroclean step isrepeated, generally in a second and completely separate tank, but underthe same operating conditions as used in the first such treatment.

(d) After another Water rinse, the part is given an acid clean treatmentin a proprietary acid salt solution, such as a mixture of sodiumbisulfite and ammonium persulfate, or regular mineral acid solution,typically for 1 to 5 minutes at 200 F., at an acid salt or equivalentmineral acid concentration of from 1 to 8 ozs./ gal.

(e) Finally, after still another thorough Water rinse, the substrate ismade the cathode in a plating tank containing a proprietary nickelelectrolyte and plated with a bright nickel deposit. Conventionally,too, a further protective thin chromium plate is deposited in asubsequent plating step.

Thus there are many possibilities for change which can and do affect theend result, some of which have been investigated and their results aregenerally predictable, but many of which cannot be anticipated orrationalized.

The present invention rests on a discovery involving modification of theconventional cycle of operations above outlined to incorporate a step ofimmersing the substrate metal part in an aqueous solution of awater-soluble organic chelating/complexing agent, or combination of suchagents. It is an important aspect of the present discovery that suchchelating/complexting solution treatment follow the soak cleaner stepand precede the electroclean step (or the first of such steps if two areemployed); or alternatively that the chelating/complexing treatment stepfollow the electrocleaner (or replace the second electroclean operationif two are employed) and precede the acid clean treatment.

When this modification is employed, as more fully described below, thedegree of improvement is so remarkable with respect to depth, clarityand uniformity of final electroplate as to lead those skilled in the artto conclude that the bright nickel plating operation has beenextensively modified, whereas this is not so.

The reason for the improvement afforded by the invention is not fullyunderstood. Therefore, without intending the following explanation to bebinding, it is postulated that much of the difficulty encountered in thenormal processing cycle of steps described above is due to reactionsbetween some of the alloying metals of the copper substrate and buffingor polishing compounds used in mechanically preparing the substratesurface. Such reactions are capable of producing trace amounts of metalsoaps and/or other insoluble metal compounds on the surface of thesubstrate. These trace amounts of insoluble compounds probably becometrapped and are not readily removed by the sequence of treatmentsheretofore normally employed. However, with the incorporation of thechelating/complexing treatment step taught by this disclosure, themodified process is quite effective in solubilizing and removing thepostulated insoluble surface film to a greater degree than has usuallybeen accomplished heretofore.

As mentioned above, there is some criticality to the order of treatmentsteps to attain the best results; for example, merely adding anotherwise satisfactory chelating/ complexing agent to the soak cleaneror to the electrocleaner treatment tank, as might appear reasonable,does not produce the same order of improvement as where a separatechelating/complexing treatment step is utilized. Presumably somecompeting reaction occurs if such a combined soak cleaner/chelator orelectroclean/chelator combination is attempted, with resulting loss ofeffectiveness of the chelating agent.

Specific chelating or complexing agents found to be useful in thepractice of the invention include the following:

(a) Tetrasodium (and potassium and ammonium) pyrophosphate.

(b) Sodium (and potassium) tripoly phosphate.

Various alkaline salts of nitrilotriacetic acid (NTA), including thesodium, potassium, lithium and ammonium salts.

(d) Similar alkaline salts of hydroxyethyl ethylenediamine triaceticacid (HEDTA).

(e) Similar alkaline salts of diethylenetriamine pentaacetic acid(DTPA).

(f) Similar alkaline salts of ethylenediamine tetraacetic acid (EDTA).

Similar alkaline aminoacetic acid.

(h) Similar alkali metal salts of gluconic, tartaric, heptonic, oxalicand citric acids.

(i) Amines such as ethylenediamine, triethanolamine, and diethylenetriamine.

(j) Trisodium salt of aminotrimethyl phosphoric acid.

Certain combinations of the foregoing agents, which are not incompatiblein respect to the substituent groupings, are quite effective andsometimes preferred. Thus a combination of sodium gluconate and thetetra sodium salt of ethylenediamine tetraacetic acid is especiallysatisfactory.

With regard to operative conditions of use of the chelating/complexingsolution, it has been found that very dilute concentrations of theagents in aqueous solution produce substantial improvement. Whileminimum effective concentration will of course depend on the particularagent or combination of agents employed, as well as the factors of timeof immersion and temperature of the solution, in general at least about0.002 molar concentration is required. It is preferred, however, forpractical operations to employ a higher concentration, usually in therange of about 0.01 molar to 0.02 molar. Some better understanding ofthe concentration factor can be obtained from the specific examplesgiven below for illustration purposes. There is apparently nocriticality as to maximum concentration of the chelating/complexingagents in solution so far as operability is concerned. Rather, the upperconcentration limit is determined by solubility and economic factors.

The temperature of the chelating/complexing solution, and the time ofimmersion, are naturally interrelated in a generally inverse order. Forindustrially useful operations, a minimum temperature of at least 45 F.is required, while the maximum is just below boiling. The preferredcondition is within the range of 125 F. to 155 F. This corresponds withan immersion time of about 30 seconds to one minute, which again isdetermined by practical conditions. Immersion times may however be saltsof N,N-dihydroxyethyl varied from as little as 5 seconds up to at least10 minutes, with appropriate adjustment in solution temperature andconcentration.

The parameters within which successful practice of the invention ispossible are illustrated by the following examples of some specificpreplating preparation and plating cycles.

Example 1 The conventional preparation and plating cycle of steps abovedescribed is followed, with the exception that the secondelectrocleaning operation, step (c), is replaced by treatment of themetal part in an aqueous solution of sodium gluconate and thetetra-sodium salt of ethylenediamine tetraacetic acid. An approximateweight ratio of 2 parts gluconate salt to 1 part EDTA salt (tetrasodium)is employed, with a total concentration of the two of about 2 ounces pergallon of solution. The foregoing solution represents the optimumconcentration condition and some variation is possible. In general, thepreferred range of complexer bath composition consists of about 1% to10% by volume of a concentrate comprising 60% by Weight of water, 30% byweight of sodium gluconate and 10% by weight of the tetrasodium salt ofEDTA.

The temperature of the solution is maintained at F. and the brass partsto be plated are immersed in this for about 30 seconds. After thischelator/complexer step, the parts are given the standard acid clean, asspecified in the conventional process, followed by nickel plating andthe deposition of a thin protective chromium plate.

The resulting parts exhibit a specular surface free of haze, clouds andstriations, and the electrodeposited metal has excellent adherence tothe brass without any evidence of blisters.

As a variation of this procedure, the chelator/complexer solution may bemaintained at room (75 F.) temperature, in which case the parts areimmersed in it for about 1 minute. At still lower temperature (45 F.) ofthe chelator/complexer, it is still effective but the time of immersionmust be increased substantially, generally to around 10 minutes.

Example 2 The procedure of Example 1 is duplicated with the exceptionthat the chelator/complexer solution contains only sodium gluconate, ata concentration of 4 ounces per gallon. At a solution temperature of 120F., good clear nickel plated deposits are obtained using a 30 second dipin the chelator/activator solution, and still better results areobtained if the immersion time is increased to 1 minute.

Example 3 The same procedure was followed as in Example 1, except thatthe chelator/complexer solution consisted of a 5% (volume) aqueoussolution of Dequest 2006. This is a chelating agent consisting of aminomethyl sodium salt of phosphoric acid, produced by Monsanto Co. At asolution temperature of 120 F. and an immersion time of 30 seconds,haze-free, adherent nickel plated deposits are uniformly obtained on thebrass substrates.

Substituting Dequest 2001 for the material above does not give whollysatisfactory results because of attack of the brass. This material is anacid form of Dequest 2006.

Example 4 The procedure of the foregoing example is repeated using anaqueous solution of the tri-sodium salt of nitrilotriacetic acid. At aconcentration of 4 oz./gal. and a solution temperature of about 120 F.,excellent plate deposits are obtained with an immersion time of 30seconds in the chelator/complexer bath.

At a solution temperature of F., 30 second immersion, the resultingplated deposit is clear and bright, even though some tarnishing of thebrass substrate appears. Decreasing the solution temperature to 75 F.,still retaining a 30-second immersion time also gives excellent resultsin respect to plated deposit clarity, brilliance and adhesion. Notarnishing of the brass is apparent in this case.

Modification of the concentration of the chelator concentration, as byreducing it in half, does not noticeably affect the finish where thesolutionis maintained at 120 F. At lower temperatures, some increase inimmersion time appears beneficial. Somewhat less tendancy for tarnishingof the substrate brass surface is noted at the lower pH conditionresulting from decrease in the concentration of the agent.

Example 5 The procedure is repeating using Hampshire DEG as thechelator/complexer agent. This is an anionic liquid sequestrant sold byHampshire Chemical, Div. of W. R. Grace & Co., Nashua, NH, and consistsessentially of sodium dihydroxyethyl glycine. A 5% (vol.) solution ofthis agent in water, at a temperature of 120 F., gives excellent resultson 30 second immersion cycle. Reducing the concentration to around 2%(vol.) at the same temperature still provides entirely satisfactoryfinish in the nickel plate. At the lower concentration but raising thebath temperature to 160 F., some tarnishing of the brass substrateappears, however the final nickel plate finish is still excellent.

Example 6 The procedure is repeated using diethylenetriamine at aconcentration of 2% (vol.) in aqueous solution at 120 F. for 30 seconds.A satisfactory final nickel finish is obtained which, however, does notappear quite as bright and clear as in some of the preceding examples.Operation of this solution at higher temperature, e.g. 160 F., showssome increase in tarnish in the brass substrate prior to plating butdoes not appear substantially to affect the final nickel finish.

Example 7 The several alkali metal salts of ethylenediamine tetraaceticacid also produce satisfactory results when used as chelator/complexeragents in the novel process. Hampene-Na Hamp-ene-Na and Hamp-ene-Na (allHampshire Chemical, Div. W. R. Grace & Co.), the various hydrated sodiumsalts of EDTA, as well as Hampcue-215 the technical tetrasodium salt,all produce a bright clear finish when used in concentrations of from 2to 5 ounces/gal. at 120-130 F.

Example 8 Diethylene triamine penta acetic acid pentasodium salt,available commercially as Hamp-ExSO, is also suitable for use as achelator/complexer agent in the process of the invention. The sameconcentration range (approximately 2 to 5 oz./gal.), at the sametemperature conditions (120-130 F. preferred, but operative up to atleast 160 F.) for similar immersion times give clear, bright finalnickel plated finishes.

Example 9 Another suitable agent is the trisodium salt of N-hydroxyethylethylenediamine triacetic acid. This is available commercially asHamp-OL Crystals and the Hamp-OL 120, a liquid. Use of these in amountsof 2 to 5 oz./gal. of the crystalline material, or 5% (vol.) of theliquid, produce a bright, specular finish in the final nickel platedbrass substrate.

Example 10 Sodium tripolyphosphate, tetrasodium pyrophosphate, sodiumhexametaphosphate, and to somewhat lesser extent, trisodiumorthophosphate, can be used alone as the chelator/complexer in thepractice of the invention. Satisfactory results are obtained atconcentrations of 2 to 5 oz./gal. It is usually necessary with these tooperate at higher temperatures, generally at least 160 F. to getcomparably good results.

Example 11 The alkaline salts of carboxylic acids in addition togluconic are also operative. Sodium tartrate, sodium heptonate, sodiumoxalate and sodium citrate each performs similarly to the gluconate.These may be used in combinations with each other or with othercomplexing agents mentioned above. The concentrations and otherconditions of operation are generally the same as above described.

In all cases a more uniform result is obtained over a longer period ofuse of the chelator/complexer bath where a thorough water rinse of thebrass parts is provided ahead of the complexing agent. It is alsopreferable to have a water rinse immediately following the complexingagent treatment.

In each of the examples above, the chelator/complexer was used after anelectroclean treatment. The procedure will also work where thechelator/complexer step immediately follows the soak clean (and a waterrinse) and precedes the electroclean step. The conditions as toconcentration, temperature and time remain as before.

What is claimed is:

1. In the method of nickel plating a substrate surface composed of acopper alloy containing Zinc, tin, lead or combinations thereof, whereinthe plating cycle ineludes immersing the substrate successively inconventional soak cleaner, electrocleaner and acid clean solutions,followed by electrodeposition of the nickel, the improvement whichcomprises: immersing the substrate in an aqueous solution of achelating/complexing agent after the soak cleaner but before the acidcleaner steps in said cycle.

2. The method of claim 1, wherein the chelator/complexer immersion stepfollows the soak clean but precedes the electroclean step.

3. The method as defined in claim 1, wherein the complexer immersionstep follows the electroclean but pre cedes the acid clean step.

4. The method of claim 1, wherein the substrate is immersed in thechelator/complexer solution for a period of 5 seconds to 10 minutes at asolution temperature of from 45 F. to just below boiling, said complexersolution being from at least about 0.002 molar up to the limit ofsaturation in respect to said chelator/complexer agent.

5. The method of claim 4, wherein the period of immersion in saidcomplexer solution is about 30 seconds to 1 minute at a solutiontemperature of about to F. and the concentration of complexing agent isabout 0.010 molar to 0.020 molar in said chelator/compleXer agent.

6. The method of claim 4, wherein said aqueous concentrate of saidcomplexer consists of about 1% to 10% by volume of a concentratecomprising 60% by weight of water, 30% by weight of sodium gluconate and10% by weight of the tetrasodium salt of EDTA.

7. The method as defined in claim 1, wherein said complexing agent is amember selected from the group consisting of A. sodium, potassium andammonium salts of pyrophosphates and sodium and potassium salts ofpolyphosphates B. sodium, potassium and ammonium salts of lower alkylcarboxylic acids C. sodium, potassium and ammonium salts of amino andhydroxy-amino substituted lower alkyl carboxylic acids, and

D. lower alkyl substituted amines.

8. The method as defined in claim 1, wherein said complexing agent is amember selected from the group consisting of A. sodium, potassium andammonium pyrophosphates and sodium and potassium tripolyphosphates;

7 8 B. sodium, potassium and ammonium salts of gluconic, ReferencesCited tartaric, heptonic, citric and oxalic acids; FOREIGN PATENTS C.sodium, potassium and ammonium salts of nitrilo- 1,421,965 11/1968Germany 204 29 triacetic, hydroxyethyl ethylenediamine triacetic,diethylene triamine pentaacetic, ethylenediamine tetra- 5 JOHN MACK,Prlmary Examiner acetic and dihydroxyethyl aminoacetic acids; W. I.SOLOMON, Assistant Examiner D. ethylenediamine, tricthanol amine anddiethylene triamine, US. Cl. X.R.

and mixtures thereof. 10

